The invention lies in the field of processes for the recovery of aluminum from its ores using crystallization procedures.
The present invention is an improvement in the process disclosed in the above-referenced patents, this latter process including the following process steps as illustrated in the flow diagrams of the referenced patents.
1. roasting particulate alunite concentrate to remove water of hydration; PA1 2. removing sulfur compounds and alkali metal compounds, including potassium sulfate, from the roasted ore, either by the procedure of U.S. Pat. No. 3,890,425 in which the roasted ore is subjected to a reducing roast, and then to an oxidizing roast followed by leaching with a solvent, such as water or an alkaline solution, or by the process of U.S. Pat. No. 3,890,426 in which the roasted ore is leached in a solvent, such as ammonium hydroxide or other alkaline solution; PA1 3. recovering potassium sulfate from the leach liquor of step (2) by heating the leach liquor to elevated temperatures to permit economic crystallization followed by crystallization as disclosed in U.S. Pat. No. 3,890,426; PA1 4. digesting the residue from the leaching step with caustic solution to convert aluminum values in the residue to soluble aluminates; PA1 5. precipitating silica from the resulting solution of step (4) to leave a solution of aluminates and a residue known as a "desilication product" consisting essentially of sodium aluminum silicate, along with other impurities; PA1 6. precipitating aluminum values from the solution of step (5); and PA1 7. recovering product aluminum from the aluminum values of step (6).
Both of the alternatives of step (2) above are included in the expression used herein "roasting and leaching to remove water and compounds of sulfur and alkali metals."
The sales price of a commercial grade metal recovered from an ore is dependent upon economic factors related to the process for recovering the metal from its ores. Elimination of steps, converting the process to a continuous process, elimination of reagents, use of lower temperatures, etc., are examples of expedients which reduce the expense of the process, and therefore reduce the expense of recovering the metal. In the present invention, the improvement in the process lies in a procedure by which potassium sulface can be economically crystallized from the potassium sulfate liquor with the use of lower temperatures so that a saving in the heat required is achieved, or conversely, more K.sub.2 SO.sub.4 is recovered at the same temperature with the result that more k.sub.2 SO.sub.4 is recovered with the same amount of heat.
In the procedure as disclosed in the above-referenced patents, the efficiency of the potassium sulfate crystallization from the leach liquor is limited by the chemical solubility constraint of potassium sulfate. In order for the crystallization to be efficient, the concentration of potassium sulfate in the leach liquor must be very high and this can only be achieved by the use of high temperatures to, in effect, produce a super-saturated solution. In view of the large amount of leach liquor which must be processed in the production process of recovering aluminum from alunite ore, the required heat is a significant expense item in the process.
Accordingly, it is a principal objective of this invention to provide an improvement in the procedure for recovering potassium sulfate from the leach liquor of the basic process in which the heat required for the crystallization step is significantly reduced.